Any Windows OS with 32-bit will do the job.

Awesome! I'm having a bit of trouble finding a XP iso that is still actively seeded... Which version of XP should I go with?

Yeah that's the one I've been using, it works if I'm transferring game .roms and .savs, I had no problem loading Pokemon Red and a completed .sav file to the cart, but then tried to upload LSDJ and an album that was on a .sav file, and it corrupted the midi functions and songs

What about the Mac/Linux driver? I've used it in Linux and its fantastic. No multirom support though AFAIK.

Edit: Here's the link.
http://lacklustre.net/projects/ems-flasher/

I uploaded the disc contents for the 64M EMS Smart Card to my dropbox along with some other flash cart programs:
https://www.dropbox.com/sh/lluwt28tnpgmfgq/qIgMrEq_Cg

If you need help with installation, let me know!

My bad, they're EMS carts. I've tried Parallels Desktop, but now that I think about it, I never downloaded the Windows OS ISO and couldn't use the program. I'll find the Windows XP disc image and see if I can get Parallels up and running. Thanks!

Which ones are the USB Smart Carts?

EMS?
Smartboy?

I have a 2011 MBP w/ Mountain Lion and I run Windows XP on "Parallels Desktop" whenever I need to transfer SAVs and ROMs from my flash carts to my computer.

Can someone point me in the direction of a good Windows emulator for Mac? Or a good Mac App that works with USB Smart Carts? I've tried using the command prompt method (there's another name but can't think of it) to flash .roms and .savs to my carts, but the files always get corrupted slightly to where some of the functions (such as LSDJ MIDI Out) will not work.